Microwave radiation detectors are widely used in intrusion detection systems, e.g. burglar alarm systems. A typical microwave detector includes a microwave transmitter which transmits microwave energy, typically microwave energy at a frequency on the order of 10 GHz, into a supervised region such as a room, and a microwave receiver which receives microwave energy reflected from the region. The transmitted microwave energy may be continuous or pulsed with a constant duty cycle, for example a duty cycle on the order of 1:100. The receiver provides an electric output responsive to changes in certain parameters, e.g. frequency, of the microwave radiation. The receiver output is amplified and processed to determine whether or not changes in the reflected radiation, such as frequency shifts, indicate an intrusion situation. If the amplified receiver output meets predetermined criteria, an intrusion alarm or some other indication is activated.
It should be noted that microwave intrusion detection is generally based on detection of changes of the received signal parameters and is generally not dependent on the absolute values of the received signal parameters. Therefore, for example, when no changes are detected in the received signal parameters, the detection system will generally not detect an intrusion.
It is well known that the diodes used in the transmitters and/or receivers of existing microwave detectors, e.g. Gunn-type diodes, are susceptible to manufacture/maintenance/handling effects, such as relatively low-level electrostatic discharge, as well as to natural effects such as changes in temperature and/or humidity. Thus, many existing microwave detection systems include auto-supervision, i.e. self-test, devices and/or modes of operation.
Techniques for supervising and monitoring microwave alarm systems are described, for example, in U.S. Pat. No. 5,093,656 and U.S. Pat. No. 5,287,111. The U.S. Pat. No. 5,287,111 Patent describes a microwave detection system in which the average energy of the transmitted microwave signal is varied periodically in order to self-test the detection system. The average energy is varied by changing the amplitude, duty cycle and/or pulse width of the signal. Sensor monitoring techniques for microwave and/or other intrusion detection systems are also described in U.S. Pat. No. 4,611,197.
Intrusion detection systems combining two, different, detection technologies such as microwave detection, pyroelectric detection, acoustic detection, etc., are known in the art. For example, U.S. Pat. No. 4,660,024 describes a dual technology intruder detection system including two detection subsystems. The two subsystems use two, respective, detection technologies to detect intrusions within predefined time intervals, whereby intrusion by either subsystem activates a common alarm mechanism. Patent '024 also describes a supervisory circuit for detecting malfunctions in either of the subsystems. When a malfunction is detected in one of the subsystems, the other subsystem remains operable.
Many microwave detection systems, such as dual infrared and microwave systems, include one or more lamps, generally LEDs, which light up periodically for signaling purposes. The LEDs give rise to signals which may be interpreted by the microwave detection system as indicating the presence of an intruder. In order to avoid false alarms, prior art systems ignore signals received during the transition time of the LED, and delay detection until after the transient signal has settled. As a result, such systems may fail to recognize actual intruder motion, for example, if the motion occurs only during the detection delay.
U.S. Pat. No. 5,450,062 which is incorporated herein by reference, suggests switching off the LED for a period substantially longer than a sampling period of the microwave system. This requires a continuous feedback connection between the microwave system and the LEDs.